This invention is directed to machine tools and more particularly, to cutting machines and tools using a machine frame formed with an outer beam or strut framework with a tool mounted on servostruts for machining within the strut framework.
The above-identified patent application describes in detail the preferred strut framework formed of triangular sections or sides joined at six nodes or corners in the form of an octahedron. The present invention is not limited to an octahedron; but this is the preferred embodiment for reasons given hereinafter. The tool is mounted on a servostrut support, or platform, which has the outer ends of the servostruts connected by pivotal sockets and spherical heads to the nodes of a triangular section of the framework. The preferred servostrut support is a hexapodal strut platform or support often called a Stewart platform.
The combination of the hexapodal tool holder mounted on the nodes of the octahedral framework allows a precision economic machine that can machine hard metals to micron accuracies. The framework is preferably self-contained in that it needs no foundation for stability. It is also simple to build because it essentially is formed of eight, triangular sections or faces joined together at six corner junctions or nodes. A characteristic of the octahedron framework is that when forces are applied at its corners, there are no bending moments with all of the forces converted to axial ones in the strut-like members of the framework so that deflections are directly proportional to the lengths of the members and not the cube of their lengths; thereby providing a stiff structure with a minimum of framework material. For example, it is thought that a machine of this type can be built weighing only 40,000 lbs. versus a weight of 200,000 lbs. for a machine of conventional design. With such an octahedral framework, it should be possible to have a stiffness of 2,000,000 pounds per inch at the spindle, and a machining precision of one micron or less. The octahedron is one of the best ways to achieve maximum possible stiffness for a given mass of metal; and hence, is economical with respect to the amount of metal used for the machine frame.
With conventional orthogonal machines originally developed for manual control, if a straight line motion was required, the machine column, bed or tool slide was constructed to provide a straight line motion. For each of these straight line motions in the X, Y and Z directions, a different machine assembly was constructed and fitted to provide the straight line movement needed. Each of these machine assemblies had a different stiffness and they often were substantially different. Because of the open kinematic structure, the compliances or deflections due to each of the members are additive so that the overall machine stiffness is substantially less than that of its weakest members.
Another shortcoming of conventional five-axes machines is that the fitting and alignment procedures for initial set-up and for adjustment when parts begin to be out of tolerance is time-consuming and a basically manual operation. While errors in the X, Y and Z directions could be adjusted electronically or by servomotor controls, errors in pitch, yaw and roll about these respective axes could only be corrected manually with time-consuming adjustment, alignment, scraping, and special fitting techniques.
In conventional machines, under CNC control, complex machine motions are developed by driving machine elements along each of the X, Y and Z axes separately. Unlike these machines, the present invention uses a hexapodal platform that has six extensible and retractable struts, all of which must move simultaneously in parallel while sharing by the load in carefully coordinated moves. Adjustments for pitch, yaw and roll may be made by electronic offsets, in many instances, without the time-consuming manual and physical adjustments necessary for conventional, orthogonal cutting machines.
In developing the hexapodal platform and mounting it on a triangular panel of the octahedral framework, it has been found that it is not possible to locate both of the bearing mounts for the servostruts in a small corner block at a corner node of the octahedron; so that the forces from the struts would produce only axial-directed forces without bending moments. The remaining beams of the octahedron without these bearing mounts can be joined together in smaller, nodal corner blocks so that only axial forces are applied to each other without bending moments. While the pairs of bearing mounts for the servostruts are brought as close together as possible, these large, nodal corner blocks at the corners of the triangular servostrut section experience significant bending moments for certain movements of their associated servostruts, such as when one servostrut is pulling while the other servostrut is pushing. Thus, there is a need to overcome this problem of bending moments in the triangular servostrut section.
Another shortcoming of existing cutting machines is that a large amount of peripheral equipment is used with them to accomplish such tasks as tool changing, workpiece loading and unloading, and workpiece clamping and unclamping. While various equipment is provided to perform such tasks, for example, an automatic tool changer, such equipment adds considerably to the cost and size of the machine.
Also, present orthogonal machines usually require manual operators to calibrate or set-up the tool to within the desired tolerances and precision locations. Thermal changes, vibrations, wear or deflection of the tool, or other changing conditions with respect to the tool fixture, pallet or workholder may cause a part to be made out of tolerance. If such a part were out-of-tolerance after a rough cut and before a finish cut, or if such positional variations are detected before rough cutting, then the orthogonal machine could be recalibrated manually. Usually, without manual intervention, there is no practical way to recalibrate the orthogonal machine to assure that the part will be made to tolerance and no easy way for the machine itself to inspect the part to see that it is made to tolerance.